Proposition 4.4 (Section 8.1.10 in [1]}, see also Proposition 1.14 in [2]}) 
Let \(A\)  and \(B\)  be two simple perverse sheaves in \({\mathcal {D}}_{G_{{\mathbf {V}}}}^{b,ss}(E_{{\mathbf {V}}})\) . Then
\(\lbrace A,B\rbrace \in \left\lbrace \begin{array}{cc}1+v\mathbb {N}[[v]] & \textrm {if A\simeq D(B)} \\v\mathbb {N}[[v]] & \textrm {otherwise}.\end{array}\right.\) 

